document Rust prerequisites

[cfm]

Describe the change

After freedomofpress/securedrop-client#1718, a full Rust toolchain is a prerequisite for running the SecureDrop Client via its ./run.sh, even if you don't intend to do any work on securedrop-proxy.

How will this impact users?

Developers working on the SecureDrop Client MUST have a full Rust toolchain installed.

User Research Evidence

freedomofpress/securedrop-client#1718 (comment)

[legoktm]

In freedomofpress/securedrop-client#2294 @deeplow flagged that we don't document the libssl-dev (or openssl-devel on Fedora) dependency needed for the proxy's Rust code.